1. Field of the Invention
The present invention relates to a method for preventing polymerization of (meth)acrylic acid and/or the ester thereof during production thereof, and in particular to a method for effectively preventing generation of polymers, which often causes problems during distillation of liquids containing (meth)acrylic acid and/or the ester thereof.
2. Description of the Related Art
(Meth)acrylic acid and the ester thereof have been hitherto produced by various processes. Acrylic acid is prepared, for example, by catalytically oxidizing propylene and/or acrolein (hereinafter, referred to as “propylene and the like”) with a molecular oxygen-containing gas in gas phase, collecting an aqueous acrylic acid solution obtained by bringing the reaction products (reaction gas) obtained into contact with an absorption liquid, and then distilling the aqueous acrylic acid solution. For production of high-purity acrylic acid suitable as an industrial product, it is desirable to remove impurities contained in the aqueous acrylic acid solution, such as formaldehyde, furfural, benzaldehyde, propionic acid, formic acid, acetic acid, maleic acid, and the like as much as possible by distillation or other methods, but because the relative volatility of acrylic acid to water or acrylic acid to acetic acid is small, it is difficult to obtain high-purity acrylic acid in a simple distillation process, and thus these impurities were removed by various distillation methods combined. For example, such an aqueous acrylic acid solution is first distilled together with an azeotropic solvent (azeotropic distillation) to remove water and low-boiling components, giving an acrylic acid-containing solution containing high-boiling impurities and polymerization inhibitors, and then the high-boiling impurities and polymerization inhibitors are removed from the acrylic acid-containing solution by various other distillation methods. In addition, additional distillation, crystallization or the like is conducted as needed for removal of the trace amounts of impurities and production of high-purity acrylic acid.
In the distillation process, the vapor generated by distillation is withdrawn from the top of the distillation column and condensed, and part of the condensate is circulated as the reflux liquid into the distillation column from the top thereof. As many polymers are generated in incidental facilities of the column such as the condenser, condensate tank, and the like, it is required to suppress generation of polymers in these incidental facilities of column for more stabilized operation thereof. In addition, it is needed to maintain the concentration of polymerization inhibitors added into the distillation column at a certain level or higher for securing the potential of the polymerization inhibitors, and hence to add the polymerization inhibitors continuously into the distillation column, as the polymerization inhibitors are also distilled off continuously from the distillation column.
Various methods, for example, of raising the concentration of polymerization inhibitors in the distilling solution (feed solution) and of directly supplying the polymerization inhibitor into the distillation column, are adopted to solve the problems above (e.g., Japanese. Examined Patent Publication No. Hei7-72204 and Japanese unexamined patent application No. Hei11-236352). However, even though capable of suppressing polymerization in the distillation column, these methods cannot suppress generation of polymers in the condenser if the polymerization inhibitors are removed from the bottom of the column. Accordingly, a polymerization inhibitor is added to the condensate for raising the concentration of polymerization inhibitor in the condensate and thus for preventing generation of polymers in the incidental facilities of column, and the condensate is circulated into the distillation column for preventing polymerization in the distillation column (e.g., Japanese unexamined patent application No. 2001-348360).
However, if the polymerization inhibitor is added to the condensate in the amount sufficient for preventing polymerization in the distillation column, the concentration of the polymerization inhibitor in the condensate becomes higher than the amount required, and thus such methods also raised the problem of additional cost, for example, for installing an additional distillation process for removal of the polymerization inhibitor.
For example, in the case of a high-boiling impurity separation column, which has a relatively high distillation temperature, the concentration of polymerization inhibitor should be raised to a relatively high level for effective prevention of polymerization in the distillation column and the concentration of polymerization inhibitor in the condensate should also be raised. However, because such polymerization inhibitors should be removed before the final purified product is obtained, such increase in the concentration of polymerization inhibitors in the condensate caused the problem that it demands an additional distillation process, or expansion of the distillation column facility for removal of the polymerization inhibitors.
Alternatively, in the case of rectification column in the final distillation process, if a polymerization inhibitor higher in polymerization-inhibiting potential, which exerts a greater effect even with a small amount thereof, such as phenothiazine or an N-oxyl compound, is added to condensate and part of the condensate is circulated into the column for prevention of polymerization in the distillation column, the incidental facilities of column and of the condensate for the purpose of maintaining the purity of the condensate obtained by distillation, the purified product from such a process may have a purity satisfying the requirements for the product, but the use of such a polymerization inhibitor causes the problem of inferior quality due to coloring of acrylic acid, as phenothiazine or the N-oxyl compound is a colored polymerization inhibitor. Although addition of a polymerization inhibitor less coloring, such as hydroquinone monomethylether or the like, is examined for prevention of decrease in quality due to coloring, such a polymerization inhibitor should be added in a greater amount due to its inherent low polymerization-inhibiting potential for prevention of polymerization in the column, and thus caused the problem of lower purity of the final purified product. In this manner, it was difficult to maintain the purity and quality of the purified product at a certain level or higher and simultaneously to suppress polymerization sufficiently in the distillation column during distillation in the final rectification column, and thus it was frequently required to discontinue operation of the distillation column and wash and remove the polymers generated in the distillation column.
The present invention has been accomplished taking into account the circumstances above, and an object of the invention is to provide a method for suppressing generation of polymers in a distillation column used in the distillation process of (meth)acrylic acid and/or (meth)acrylic ester, and suppressing polymerization of the condensate obtained by distillation in the incidental facilities of column such as condenser, condensate tank, and the like.
In particular, an object of the present invention is to provide a method for maintaining the purity and quality of (meth)acrylic acid and/or the ester thereof at a certain level or higher, suppressing generation of polymers in a distillation column, and suppressing polymerization of the condensate obtained by distillation in the incidental facilities of the column during distillation in the final rectification column.